CHAPTER VIL COMPOSITION AND UTILITY OF FLUXES. The object of fluxing furnaces and cupolas is to give fluidity to the non-metallic residuum of the iron ore and the ash of the fuel, to carry it out of the furnace or cupola in the form of slag. While this is an important function, there are certain chemical compositions that can exist in fluxes which best assist in obtaining desired restilts, similar as there are certain chemical constituents necessary in ores to obtain the brands or grades of iron desired. All fluxes should be as free of earthy matter as possible, since such retards their action. High silica and sulphur are likewise objectionable. The element most essential in a flux to aid the creation of slag is lime. This is found in various substances, as in marble, spalls, oyster and clam shells, limestone, chalk, dolomite, calc-spar, fluor-spar, and felspar. Magnesia largely serves the same end as lime, but less of it is required. About two of the former is sufficient, where three of the latter would be required. Dolomite contains more magnesia than any other class of limestone, and is often called magnesia limestone and generally contains about 55 per cent of calcium carbonate and 40 per cent of magnesium carbonate, with the rest largely silica, oxide of iron, and alumina.stic, and causing it to give way more easily from the walls of a furnace. It .generally takes from five to ten hours for stock to work down from the top to be tapped out as iron.